Molecular biology advances in the last decade gave great promise for the introduction of new, sensitive technologies to identify various analytes in test specimens, including the ability to diagnose cancer, infectious agents and inherited diseases. Clinical molecular diagnostics depend almost exclusively on restriction enzyme analyses and nucleic acid hybridization (Southern and Northern blots) (Meselson and Yuan, 1968, Southern, 1975). Clinical tests based on molecular biology technology are more specific than conventional immunoassay procedures and can discriminate between genetic determinants of two closely related organisms. With their high specificity, nucleic acid procedures are very important tools of molecular pathology. However, nucleic acid procedures have limitations, the most important of which are the procedures consume time, they are labor intensive, and have low sensitivity (Nakamura 1993).
In any sample, the number of protein molecules of one kind is usually several times higher than the corresponding mRNA, and several hundred times higher than the number of genes encoding them. Using antigen-specific antibodies is a routine procedure in the modem diagnostic industry, although antibody development and purification usually require laborious work. The specificity of tests based on monoclonal antibodies depends on the capacity of antibodies to differentiate between antigens, and might approach the specificity of tests based on nucleic acid hybridization. The sensitivity of these tests, however, is routinely significantly lower than tests based on nucleic acid hybridization, even though the number of protein target molecules in each cell is relatively higher than the nucleic acid molecules corresponding to them. It is desirable to use proteins as the targets in diagnostic tests because of their abundance.
Thus, a need exists for improved diagnostic and analytical methods to detect the presence or absence of target molecules. A need also exists to detect non-nucleic acid analytes with nucleic acid chemistry. And, there is a need to detect protein targets with nucleic acid chemistry coupled to a amplification system.